The field of the invention relates to control systems responsive to an exhaust gas oxygen sensor for maintaining an engine's air/fuel ratio at stoichiometric combustion.
U.S. Pat. No. 4,867,126 issued to Kortge et al discloses an engine having a fuel vapor recovery system coupled between a fuel system and engine air/fuel intake. A feedback control system generates a feedback variable by integrating the output of an exhaust gas oxygen sensor. Liquid fuel injected into the engine is trimmed in response to the feedback variable in an attempt to maintain stoichiometric combustion. When the feedback variable exceeds a predetermined value, the induction of recovered fuel vapors is reduced to, allegedly, maintain operation within the feedback system's range of authority.
The inventors herein have recognized several problems with the above approach. Even when the rate of vapor flow is reduced to zero, there are certain engine operating conditions where the feedback system will induce an air/fuel transient. During engine deceleration, for example, the low rate of air induction may result in rich operation because the fuel injectors are operating below their linear range. That is, the fuel injectors will deliver more fuel than demanded when the actuating electrical pulse width is below a critical pulse width. The engine will continue to operate rich during deceleration and the feedback variable will continue to provide a lean correction without effect. When the engine throttle is restored, the lean correction provided by the feedback variable will then cause operation lean of stoichiometry resulting in engine "stumble".